1. Field of the Inventive Concept(S)
The presently disclosed and/or claimed inventive concept(s) relates generally to the reduction of carbon dioxide by heterogeneous catalysis. More particularly, but not by way of limitation, the presently disclosed and/or claimed inventive concept(s) relates to the reduction of carbon dioxide by heterogeneous catalysis with a heterogeneous hydrogenation catalyst comprising structurally frustrated Lewis pairs, wherein, for example but not by way of limitation, formic acid is produced and hydrocarbons are indirectly produced. In one non-limiting embodiment, the heterogeneous catalyst comprises hexagonal boron nitride (h-BN) having structurally frustrated Lewis pairs therein.
2. Background of the Inventive Concept(S)
Carbon dioxide (CO2), a well-known greenhouse gas, is the main product emitted by the combustion of hydrocarbons for the generation of power for many uses including electricity and transportation. In 2012, CO2 production was at a record high of 31.6 gigatons, a number that will continue to escalate as consumer demands increase. As such, it is critically important to the environment to reduce the emissions of CO2.
Existing approaches to reducing carbon dioxide emissions include sequestration, electrochemical reduction, and homogeneous reduction. However, each of these processes has specific disadvantages. For example, sequestration is limited to the space available to store CO2, electrochemical reduction is energy intensive, and homogeneous reduction requires the utilization of catalysts that are sensitive to both air and moisture. Additionally, existing approaches generally use catalysts comprising precious metals such as palladium, platinum, nickel, and rhodium, which increases the cost of CO2 reduction and depletes the supply of these precious metals. These and other factors make the presently disclosed and/or claimed process of using a heterogeneous hydrogenation catalyst, to not only reduce CO2 emissions but also produce a commercially valuable product, an attractive alternative to existing approaches.
Frustrated Lewis Pair (FLP) catalysts are potentially useful as one type of heterogeneous hydrogenation catalyst. In 2007, Stephan and his team developed a chemical system capable of releasing and absorbing molecular hydrogen using frustrated Lewis pairs. (See Frustrated Pairs, in Catalysis without Precious Metals, M. R. Bullock, Editor. 2010, Wiley-VCH Verlag GmbH & Co. KGaA. P.I-XVIII, hereby incorporated in its entirety). Stephan determined that when a sterically encumbered Lewis acid approaches a bulky Lewis base, adduct formation is hindered and gives rise to electronic “frustration”. Such frustration effectively mimics the donor-acceptor properties of transition metals. Stephan and his team demonstrated that upon exposure to 1 atm H2 at 25° C., a solution of red phosphino-borane [(C6H2Me3-2,4,6)2P(C6F4)BF(C6F5)2] transformed to the colorless zwitterionic salt [(C6H2Me3-2,4,6)2PH(C6F4)BH(C6F5)2]. Upon thermolysis at 150° C., the salt lost H2 and converted back to the original phosphine-borane substrate. Such phosphonium borates (as well as similar compounds) have been shown to successfully catalyze the hydrogenation of select imines, enamines, aldehydes, and olefins.
However, to date, only homogeneous FLP catalytic systems have been studied, i.e., FLP catalysts and the resulting reactions have previously involved the FLP catalyst being in the same phase as the reactants. For example, the homogeneous FLP catalyst is typically co-dissolved in a solvent with the reactants. Heterogeneous catalysis, on the other hand, is performed with the catalyst in a different phase from that of the reactants. One example of heterogeneous catalysis is the petrochemical alkylation process where the liquid reactants are immiscible with a solution containing the catalyst. Heterogeneous catalysis offers the advantage that products may be readily separated from the catalyst. Typically, heterogeneous catalysts are more stable and degrade much slower than homogeneous catalysts.
As detailed in the presently disclosed and/or claimed invention, heterogeneous catalysts, e.g., heterogeneous FLP catalysts, can be used to hydrogenate the carbonyl bond of carbon dioxide to produce formic acid, which as of 2013 has a commodity price of 700-1,000 USD/metric ton and can be used as a fuel or in fuel cells. Additionally, formic acid can be thermally decomposed to produce carbon monoxide and water, wherein the carbon monoxide can be further converted to a hydrocarbon fuel using Fischer-Tropsch chemistry. Given that on average a typical power plant emits 10,000 tons of CO2 per day, the ability to efficiently turn CO2 into a commodity while reducing the emissions of CO2 is a valuable alternative to processes currently available for industrial and commercial use.
In view of the foregoing, there is a need for a heterogeneous catalyst capable of chemically reducing CO2 in order to efficiently decrease the production of CO2 emissions. In particular, a heterogeneous hydrogenation catalyst having an FLP-type electronic structure would be a valuable addition to catalysts currently available for industrial and commercial use. It is to such a heterogeneous hydrogenation FLP catalyst and its method of use that the presently disclosed and/or claimed inventive concept(s) is directed.